1. The Field of the Invention
This invention pertains to production hydrogen peroxide (H2O2) by direct catalytic synthesis using hydrogen and oxygen-containing feedstreams. It pertains particularly to a supported noble metal phase-controlled catalyst having high activity and product selectivity, and which is useful for such direct hydrogen peroxide production process utilizing feedstreams having hydrogen concentration less than about 4% by volume.
2. The Relevant Technology
Demand for hydrogen peroxide product has been growing globally at about 6% annually, and in North America at about 10% annually. Such demand growth is due primarily to the environmental advantages of hydrogen peroxide usage, which upon decomposition releases only oxygen and water. Hydrogen peroxide is an effective replacement for chlorine in pump and paper bleaching, water treatment and other environmental processes, and meets the growing product demand and need for a simple environmentally friendly and cost effective process that can be located on-site for the pulp, paper and other manufacturing facilities. The hydrogen peroxide presently being produced commercially uses a known anthraquinone process which has low yields and some safety problems. Also, transportation of hydrogen peroxide from a production site to an end-user facility is an important safety issue due to the risk of explosion of hydrogen peroxide by its violent decomposition.
Many attempts have been made to produce hydrogen peroxide directly from hydrogen and oxygen-containing feedstreams, because such a process not only has potential for significantly reducing the production cost, but also provides an alternative production process which avoids the present user of toxic feedstock and working solutions. For such direct catalytic production of hydrogen peroxide, the feedstreams are hydrogen and air which are clean and environmentally harmless, and no organic solvent is used in such a direct synthesis process. The reaction medium is water, and the hydrogen peroxide formed does not contain any organic compounds. Such direct process generates no waste and is cost efficient due to its inherent simplicity and the hydrogen peroxide product can e used directly as a bleaching agent in pulp and paper processes. However, such proposed direct production technology has not yet been commercialized, as the major problems for the known processes are (1) hazardous operating conditions (with the feed hydrogen partial pressure above an explosive range), (2) low reaction rates, and (3) low catalytic product selectivity.
Although the direct catalytic synthesis of hydrogen peroxide product has attracted much attention and many patents have been issued, none of the patented processes have been commercially feasible due to low catalyst activity and the low selectivity for the production of hydrogen peroxide. Until the early 1990's, most of these patents utilized as feed gas at least 10% hydrogen in air or oxygen, which is within the explosion limits for the H2/O2 mixture. Due to increasing safety concerns, the recent approach has been to utilize feedstreams having hydrogen concentration below about 5 vol. %. However, at such low hydrogen concentration, the catalysts used must be much more active to achieve an acceptable production rate of hydrogen peroxide. Highly dispersed palladium on various support materials has been used to enhance the catalytic activity. However, the dispersion methods used have not adequately controlled the crystal phase of the palladium, and desired improvement in selectivity towards hydrogen peroxide product has not been achieved. A main problem in preparing a highly selective catalyst for hydrogen peroxide production is how to consistently control the formation of a desired metal phase such as phase 110 or 220, etc. in the catalyst.
Such known prior attempts to develop a commercial hydrogen peroxide process are described in various patents. For example, U.S. Pat. No. 4,661,337 to Brill discloses producing hydrogen peroxide by decently reacting hydrogen and oxygen at superatomospheric pressure in an acidic aqueous solution containing a suspended catalyst formed of a layer of supported noble metal such as palladium. U.S. Pat. No. 4,681,751 to Gosser discloses a catalytic process for making hydrogen peroxide from hydrogen and oxygen using as catalyst palladium on small carbon support particles in an aqueous medium containing an acid component and halide ion component. U.S. Pat. No. 4,722,458 to Gosser et al. discloses a similar process which used as catalysts Pd alone or a Pd and Pt mixture on various carriers in an aqueous reaction medium containing a bromide promoter. U.S. Pat. No. 4,832,938 to Gosser et al. discloses a similar direct hydrogen peroxide production process using platinum/palladium catalysts. U.S. Pat. No. 5,236,692 to Nagashimia et al. discloses a method for producing hydrogen peroxide by directly reacting hydrogen and oxygen in a medium containing a promoter and using a platinum group metal catalyst supported on a solid superacid carrier. U.S. Pat. No. 5,378,450 to Tomita et al. describes a process using a supported tin-modified palladium as catalyst, in a liquid medium containing no halogen ions. U.S. Pat. No. 5,399,334 to Kawakami et al. disclosed a direct hydrogen peroxide process using an aqueous medium containing an organic solvent and as catalyst palladium supported on alumina, silica or activated carbon for product selectivity ranged between 55% an 82%. Also, U.S. Pat. No. 5,338,531 to K. Chuang et al. disclosed a direct catalytic process using a feed gas containing 3.2 vol. % oxygen; 10 vol. % nitrogen and 86.8% vol. oxygen, product selectivity ranged between 38% to 100%, but deceased with increased catalyst age. The catalyst support was hydrophobic fluorinated carbon which is very expensive and has not been produced commercially in large quantity. Also, in these patented processes, the catalysts were apparently tested for only a short time.
Based on the above catalyst development activity and inferior results, it is apparent that to provide a direct catalytic hydrogen peroxide process that is commercially feasible, it is necessary to modify the noble metal structure of the catalyst so that its activity at low hydrogen concentration and partial pressure remains substantial, and also so that the catalyst is stable over sufficiently long periods of time essential for successful commercial production operations.